Deep-stops vs shallow-stops: an interesting read.

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NDL question gets interesting if you overstay the NDL. Normally you'd want a smooth transition from no-stop to non-optional safety stop (gray boxes on the RDP) to a longer 3 msw stop to a 6 msw stop and so on.

@huwporter once showed that a really silly GF setting, like 10/90 can take you from no-stop to 9 msw stop with nothing in between.
 
NDL question gets interesting if you overstay the NDL. Normally you'd want a smooth transition from no-stop to non-optional safety stop (gray boxes on the RDP) to a longer 3 msw stop to a 6 msw stop and so on.

@huwporter once showed that a really silly GF setting, like 10/90 can take you from no-stop to 9 msw stop with nothing in between.
It doesn't even need to be a silly GF setting! :) Even a commonly used setting such as 30/85 'should' jump straight from no-stop to 1@9, 1@6 immediately on exceeding the NDL, so long as you define the NDL as "the longest dive time where you would not exceed GFHi at the surface on making a direct ascent there", and GFLo as "insert a first stop at the last 3m increment before you would exceed GFLo on your ascent". (Ref. this post: On a NDL dive, which computers' NDLs are not affected by GFLo? )

Edit: actually if you do use 10/90 you can get the algorithm to go straight from no-stop to 1@12, 1@9, 1@6 :D

What this tells you IMO is a reminder that the maths we use to try and keep us safe isn't actually reflecting what is really going on in our messy biology and you should err on the side of caution. (Particularly in terms of very slow final ascent to the surface.)
 
I read the following
mentioning quote "
A large study conducted by the U.S. Navy compared the incidence of DCS in air decompression schedules for 30 minutes bottom time at 170 fsw bottom for a gas content algorithm with the first stop at 40 fsw (shallow stops) or a bubble algorithm with the first stop at 70 fsw (deep stops). The shallow stops schedule resulted in 3 DCS in 192 man-dives and the deep stops schedule resulted in 11 DCS in 198 man-dives.
"

Some questions did arise ...

Is 198 divers really that representative? large study?
navy divers vs regular divers?

To deep stop or not to deep stop?
Let's take Buhlmann ZH-L16B
dive 40 minutes at 40 meters
EAN28, DECO mixes 50 and 100

decoplanner 4.6.3 shows
GF 70/80
@12 01 min 50%
@09 05 mins 50%
@06 14 mins 100%

GF 20/80
@21 01 min 50%
@18 01 min 50%
@15 01 min 50%
@12 02 mins 50%
@09 05 mins 50%
@06 12 mins 100%
 
I read the following
mentioning quote "
A large study conducted by the U.S. Navy compared the incidence of DCS in air decompression schedules for 30 minutes bottom time at 170 fsw bottom for a gas content algorithm with the first stop at 40 fsw (shallow stops) or a bubble algorithm with the first stop at 70 fsw (deep stops). The shallow stops schedule resulted in 3 DCS in 192 man-dives and the deep stops schedule resulted in 11 DCS in 198 man-dives.
"

Some questions did arise ...

Is 198 divers really that representative? large study?
navy divers vs regular divers?

To deep stop or not to deep stop?
Let's take Buhlmann ZH-L16B
dive 40 minutes at 40 meters
EAN28, DECO mixes 50 and 100

decoplanner 4.6.3 shows
GF 70/80
@12 01 min 50%
@09 05 mins 50%
@06 14 mins 100%

GF 20/80
@21 01 min 50%
@18 01 min 50%
@15 01 min 50%
@12 02 mins 50%
@09 05 mins 50%
@06 12 mins 100%
Right off the bat you're comparing air diving/decompression to other than air decompression. Deep stops with increased O2 is much different than air. I would say if more Navy divers got DCS on deep stops with air, then it is more likely the deep stops and not the divers. IMO. Air decompression has been around for 150 years I'd think we'd have that pretty nailed at this point in time.
 
To help understand the complexity of the problem, think about 3 divers, each of whom leave the deepest portion of their NDL dive with 3 minutes of NDL on their identical computers. Draw you own conclusions.
  1. Diver #1, a deep stops enthusiast, does a 2 minute deep stop at half the maximum depth before heading toward the surface, with a 3 minute safety stop.
  2. Diver #2, a multi-level diver, extends the dive for 15 minutes at half the maximum depth, which does not exceed NDL on the computer, before heading to the surface, with a 3-minute safety stop.
  3. Diver #3, a multi-level diver, extends the dive for 15 minutes at half the maximum depth, which does not exceed NDL on the computer, and then ascends to 30 feet, where the dive is extended for another 10 minutes before heading to the surface, with a 3-minute safety stop.
The one NDL deep stops practice that I can comfortably say is questionable at best is a rare one--doing a deep stop for a few minutes and then shortening or eliminating the shallow safety stop on the belief that the deep stop makes a full safety stop unnecessary.

This is somewhat why I think tracking a SurfGF is a good idea - even for NDL divers. It isn't perfect, but IMHO, it is a "single number" that gives an ability to quantify risk.

In the examples above, if I knew the SurfGF for each of those profiles, I could tell you which one I thought had the least risk.

Regards,

- brett
 
I read the following
mentioning quote "
A large study conducted by the U.S. Navy compared the incidence of DCS in air decompression schedules for 30 minutes bottom time at 170 fsw bottom for a gas content algorithm with the first stop at 40 fsw (shallow stops) or a bubble algorithm with the first stop at 70 fsw (deep stops). The shallow stops schedule resulted in 3 DCS in 192 man-dives and the deep stops schedule resulted in 11 DCS in 198 man-dives.
"

Some questions did arise ...

Is 198 divers really that representative? large study?
navy divers vs regular divers?

To deep stop or not to deep stop?
Let's take Buhlmann ZH-L16B
dive 40 minutes at 40 meters
EAN28, DECO mixes 50 and 100

decoplanner 4.6.3 shows
GF 70/80
@12 01 min 50%
@09 05 mins 50%
@06 14 mins 100%

GF 20/80
@21 01 min 50%
@18 01 min 50%
@15 01 min 50%
@12 02 mins 50%
@09 05 mins 50%
@06 12 mins 100%
No one is bringing 50% and 100% for a 130ft dive. On top of diving a nitrox mix.
What made you use those numbers? Seems like you have an agenda to push.
Please use realistic scenarios
 
No one is bringing 50% and 100% for a 130ft dive. On top of diving a nitrox mix.
What made you use those numbers? Seems like you have an agenda to push.
Please use realistic scenarios
Who said this was a practical dive example? Just showing the difference between two GF LO Settings. Be my guest to throw in some realistic profiles if you are so keen to ..
 
I read the following
mentioning quote "
A large study conducted by the U.S. Navy compared the incidence of DCS in air decompression schedules for 30 minutes bottom time at 170 fsw bottom for a gas content algorithm with the first stop at 40 fsw (shallow stops) or a bubble algorithm with the first stop at 70 fsw (deep stops). The shallow stops schedule resulted in 3 DCS in 192 man-dives and the deep stops schedule resulted in 11 DCS in 198 man-dives.
"

Some questions did arise ...

Is 198 divers really that representative? large study?
navy divers vs regular divers?

To deep stop or not to deep stop?
Let's take Buhlmann ZH-L16B
dive 40 minutes at 40 meters
EAN28, DECO mixes 50 and 100

decoplanner 4.6.3 shows
GF 70/80
@12 01 min 50%
@09 05 mins 50%
@06 14 mins 100%

GF 20/80
@21 01 min 50%
@18 01 min 50%
@15 01 min 50%
@12 02 mins 50%
@09 05 mins 50%
@06 12 mins 100%
What is the point of your post? To show that there are some mixtures of gases and profiles for which deep slops only are a little bit worse than not having deep stops?
 
Just showing the difference between two GF LO Settings
Your example missed the reason the smaller GFLow becomes an issue: the non-negligible time at depth causes appreciably loading of the non-controlling (slower) tissues. Your example has a whopping 3 minutes of difference spent below 12 m.
 
This is somewhat why I think tracking a SurfGF is a good idea - even for NDL divers. It isn't perfect, but IMHO, it is a "single number" that gives an ability to quantify risk.

In the examples above, if I knew the SurfGF for each of those profiles, I could tell you which one I thought had the least risk.

Regards,

- brett
I agree. SurfGF is a great metric during the dive.

I think another great metric that can be used in planning is integrated GF99. It isn't just how high GF gets, but how long it stays high. By integrating the instantaneous GF over time, you get a measure of total decompression stress. Come shallow to fast at the start of deco, you add stress in your fast tissues. Cut your shallow stops short, you add stress in your slow tissues. Even after you surface, you have an initial stress level in the slowest tissues that slowly decays.

What is worse, surfacing with a GF of 85% that drops below 50% in 5 minutes, or surfacing with a GF of 80% that doesn't drop below 70% for over an hour?

Using this integrated decompression stress metric you can easily see why deep stops are a less efficient use of deco time for reducing DCS.
 

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